1. Field of the Invention
The present invention relates to an electrochemical cell.
2. Description of the Related Art
The electrochemical cell such as nonaqueous electrolyte secondary battery and electrical double layer capacitor has such characteristics as high energy density, light weight and small size, and has been used as a backup electric source of a timepiece function of a portable equipment, a backup electric source of a semiconductor memory, and the like. For these portable equipments, there are required a reduction in size, a reduction in weight and an increase in function, and there is demanded an additional high density mounting of the electrochemical cell.
Further, it is so adapted that, when the electrochemical cell is mounted to a circuit board, a reflow soldering method is generally used. The reflow soldering method is a method in which, after a solder cream has been applied to a portion, of the circuit board, to be soldered, there is mounted thereon the electrochemical cell, and the soldering is performed for every circuit board by causing it to pass within a furnace at high temperatures of 200-260° C. In order to endure the reflow soldering, a high heat resistance is demanded for the electrochemical cell.
There is known an electrochemical cell which is sealed by welding a container and a sealing plate.
This electrochemical cell is high in its sealing strength and excellent in its heat resistance because the container and the sealing plate are joined by a resistance welding and the like.
Since the electrochemical cell sealed by the welding can be made an arbitrary shape because it is not sealed by a caulking as different from a conventional coin type electrochemical, a mounting area can be effectively put to practical use.
Although the above electrochemical cell has been sealed by welding the container and the sealing plate in the atmospheric air, an internal resistance increase and a capacity decrease have become problems.
This results from the fact that, since the conventional electrochemical cell is sealed by the welding in the atmospheric air, a weld zone is oxidized and a corrosion resistance decreases.
It is explained by using FIG. 4. Hitherto, an electrode 2, an electrode 4 and an electrolyte 5 have been contained in a container 1, and a sealing plate 6 and the container 1 have been sealed by being welded in the atmospheric air. Through a weld zone 7, the container 1 and the sealing plate 6 have been joined. Since they are welded in the atmospheric air, the weld zone 7 has been oxidized and the corrosion resistance has decreased.
The weld zone 7 whose corrosion resistance has decreased contacts with the electrolyte 5 inside the electrochemical cell, so that a corroded part 8 has occurred. A voltage is applied to an inside of the electrochemical cell, where the corrosion is liable to progress and, especially if an organic solvent is used in the electrolyte, the voltage to be applied becomes high and thus the corrosion becomes severe.
By this corrosion, elements contained in the weld zone 7 are eluted into the electrolyte, and this causes a deterioration of the electrolyte and an impurity deposition onto an electrode active material, so that the internal resistance increase and the capacity decrease of the electrochemical cell are brought about. Further, it causes a decrease in sealing strength and a generation of leak of the solution. Additionally, by the decrease in sealing strength, since the electrochemical cell cannot endure an internal pressure increase at its heating time, its heat resistance decreases as well.
The 2nd problem to be solved is an oxidation of the organic solvent by a welding heat at the sealing time.
In the conventional electrochemical cell, although the container and the sealing plate have been welded in the atmospheric air, there has been a possibility that the organic solvent used in the electrolyte is oxidized and deteriorates by the welding heat and sparks generated at a welding time. The electrolyte of the electrochemical cell is used by diluting a high permittivity solvent whose viscosity is high with a low viscosity solvent. This low viscosity solvent is liable to be oxidized.